The present invention pertains to silicon-based microreactors for use in the field of microreaction engineering.
The state-of-the-art power source currently in use for portable electronic devices is the rechargeable lithium-ion battery, whose popularity has been steadily rising since its introduction in the early 1990s. From a small share of about 1.5% in 1994 in the portable electronic devices' battery market, the lithium-ion battery has enjoyed increasing acceptance, leading to a 72.1% market share in 1997 and 82% in 1998. The worldwide market for batteries in 2001 was estimated at $37.7 billion, which can be divided into rechargeable ($24.9 billion in sales in 2001) and primary (disposable) batteries ($12.8 billion in sales in 2001). Today the fastest growing product category in the battery industry is that of small, high performance batteries, such as rechargeable nickel metal hydride and lithium ion, and various primary batteries for use in portable electronic devices. This segment accounts for more than $16 billion in sales worldwide and is growing at nearly 8% per year. These battery types serve high growth markets such as notebook computers, cellular telephones, camcorders, digital cameras, cordless telephones and hand-held computers, the total sales for which were estimated to be over 600 million units in 2001.
The rechargeable batteries, however, suffer from a significant drawback: the charge carrying capacity is quickly approaching the theoretical maximum as development continues. Also, the charging cycles remain slow and tedious. At the same time, the ever-increasing functionality of portable electronic devices means that more and more power is required to operate them. Progress in portable device technology thus puts a lot of demand on the portable power sources, resulting in more frequent and tedious recharging cycles. The energy-dense PEM fuel cell, with potential energy storage densities about 7 to 8 times that of the state-of-the art rechargeable batteries, can thus be expected to provide a viable alternative to the battery as a portable power source. A major problem in the commercialization of the portable fuel cell has been difficulty and hazards involved in storage of hydrogen (either liquid or compressed) which is used as fuel.
The use of microreactors for in-situ and on-demand chemical production is gaining increasing importance as the field of microreaction engineering matures from the stage of being regarded as a theoretical concept to a technology with significant industrial applications. Various research groups have successfully developed microreactors for chemical processing applications such as partial oxidation of ammonia, nitration and chemical detection. The objective of the research effort at the Integrated Microchemical Systems Laboratory at Lehigh University's Chemical Engineering Department was to demonstrate a working microreaction system for use as a sustained source of hydrogen fuel for proton exchange membrane (PEM) fuel cells through catalytic steam reforming of methanol. The complete reformer-fuel cell unit is proposed as an alternative to conventional portable sources of electricity such as batteries for laptop computers and mobile phones due to its ability to provide an uninterrupted supply of electricity as long as a supply of methanol and water can be provided. Though considerable work already exists in the literature on the catalytic steam reforming of methanol for production of hydrogen using conventional reactors, the use of microreactors for in-situ methanol reforming is a relatively new idea. Literature on the macro-scale steam reforming of methanol includes analysis of the reaction thermodynamics for prediction of optimum reactor temperature and feed compositions, catalyst characterization studies, and experimental studies on macro-scale pilot reactors. Results obtained in the study of methanol reforming in these conventional reactors form a good background for the development of prototype microreactors for this purpose. Silicon is considered a good material for fabrication of microreactors due to the high strength of the Si-Si bonds which results in the chemical inertness and thermal stability of silicon. Well established silicon micromachining techniques commonly used in the microelectronics industry facilitate easy fabrication of microchannels and other desired features on silicon substrates thus making silicon the preferred material for prototype microreactor fabrication.